Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Generally exhaust gas aftertreatment systems include any of several different components to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust gas aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) system including a catalyst formulated to convert NOx (NO and NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3). Generally in such aftertreatment systems, an exhaust reductant, (e.g., a diesel exhaust fluid such as urea) is injected into the SCR system to provide a source of ammonia, and mixed with the exhaust gas to partially reduce the NOx gases. The reduction byproducts of the exhaust gas are then fluidically communicated to the catalyst included in the SCR system to decompose substantially all of the NOx gases into relatively harmless byproducts which are expelled out of the aftertreatment system.
Recent regulations are leading to a greater emphasis being placed on the fuel economy of engines (e.g., diesel engines). Improving fuel efficiency may lead to an increase in the amount of NOx gases emitted by the engine and being delivered to the SCR system. However, national and international emission standards limit the amount of NOx gases that may be emitted into the environment by the SCR system. Thus, while the amount of NOx gases included in the exhaust gas entering the SCR system is increasing, the amount of NOx gases which can be emitted by SCR system remains the same or is decreasing, creating a greater demand on SCR systems. This translates into smaller separation between a healthy SCR system and a failed one.